The typical epicyclic gear system has a sun gear, a ring gear surrounding the sun gear, and planet pinions located between and engaged with the sun and ring gears, and in addition, it has a carrier that provides pins about which the planet pinions rotate. A gear system so configured splits the torque transferred through the system into load paths equal in number to the number of planet pinions. This reduces the forces at each mesh and enables the system to transfer a large amount of power in a relatively compact configuration—or in other words, it has a high power density.
Often the ring gear remains fixed, leaving the carrier and sun gear to rotate. In such an arrangement power may be applied at one end of the carrier and delivered through the sun gear at a different velocity and torque. This holds true for the transmissions in wind turbines that harness the energy of the wind and convert it into electrical power.
Many epicyclic gear systems utilize a straddle-type of carrier in which the planet pinions rotate between two walls of the carrier on pins that extend between the walls, each being anchored at both of its ends in the walls. When torque is applied to the carrier at one of the end walls, the carrier will undergo a twist-like distortion, called carrier wind up, that skews its pins with respect to the sun and ring gears. This disturbs the mesh between the planet pinions and the sun and ring gears.
An epicyclic gear system in which the planet pinions are supported on and rotate about so-called “flexpins” mitigates the skewing. In this regard, a flexpin for a planet pinion at one end is anchored in and cantilevered from the wall of a carrier of which it is a part. The other or remote end of the flexpin has a sleeve fitted to it, with the sleeve extending back over but otherwise spaced radially from the flexpin. The sleeve forms part of or carries a bearing that supports one of the planet pinions. At the carrier wall the flexpin bends in one direction circumferentially relative the main axis of the system and at the opposite end bends circumferentially in the other direction, so that the sleeve remains parallel to the axis. In other words, flexpin technology employs a double cantilever to offset the skewing that would otherwise occur and thereby restores alignment at the meshes between the planet pinions and the sun and ring gears. See U.S. Pat. No. 7,297,086 and U.S. Pat. No. 6,994,651, which are incorporated herein by reference, for a further discussion of flexpin technology.
While a carrier that utilizes flexpins to support its planet pinions can have a single end wall to support the flexpins, the number of flexpins—and planet pinions as well—may be doubled by utilizing two end walls with flexpins fitted to each. See WO 2007/016336.
Irrespective of whether a carrier has flexpins mounted on a single wall or spaced apart walls, each flexpin must be anchored firmly in or to the carrier wall from which it projects and is cantilevered. For example, a simple interference fit may retain the flexpin as depicted in U.S. Pat. No. 6,994,651. Then again, the carrier wall may have a tapered bore and the flexpin a tapered end that fits into the bore and indeed beyond where it is provided with threads over which a nut threads. When the nut is turned down against the carrier wall, it draws the pin snugly into and secures it within the tapered bore, all as depicted in U.S. Pat. No. 7,056,259. Some flexpins have flanges along which they are anchored to the carrier wall with machine screws as depicted in WO 2007/016336.
The sleeves that extend back over the flexpins and provide or support the bearings on which the planet pinions rotate must be attached firmly to the remote ends of the flexpins. An interference between each sleeve and the remote end of its flexpin supplemented by a weld will work. Also, the sleeve and flexpin may be formed integral at the remote end of the flexpin. See U.S. Pat. No. 7,056,259. One requires welding metals that are difficult where the sleeve is case hardened. The other requires a complex machining operation.
Sometimes a planet pinion or its bearing becomes damaged. This typically requires replacement of the entire transmission of which the pinion or its bearing is a mere component. But replacing an entire transmission is not easily achieved and is costly, particularly when the transmission forms part of a wind turbine mounted high above the ground or off shore above the sea.